The present invention relates to a hip prosthesis that has a femoral component including a stem shaped with a parabolic outer surface and a straight dorsal indentation that extends in a straight line along its length, but has different depths. When inserted into the femoral canal it provides optimal transfer of forces to the bone. Anatomically, there is little correlation between the morphology of the diaphysis and the structure of and loads on the metaphysis. However, the physiological laws of force transfer that dictate that the bone will be most strongly reinforced at the locations where the strongest loads are encountered do apply. The loading of the bone is reflected in its structure. These principles can be used in designing a prosthesis, in particular in taking the prosthesis interface into account. It has been found that the best long-term results are not obtained with a prosthesis that most closely duplicates the compact exterior geometry of the bone (custom-made prostheses), but rather with one that possessed the reinforcing structures of the bone in an optimal manner (Draenert, K., et al., 1999, Manual of Cementing Technique, Berlin, Heidelberg: Springer). The object of this invention is to provide the reinforcing structures in the proximal femur, to apply the load to the bone proximally, and to achieve a maximum degree of rotational stability.
The design of conventional stems of femural components tends to duplicate the frontal projection of the femur. This is true both for cemented components and for uncemented anchored designs: see Charnley, J., (1960), Anchorage of the Femural Head Prosthesis to the Shaft of the Femur, J. Bone and Joint Surg. B42: 28-30, or also Zweymxc3xcller, K. A., et al., (1988), Biologic Fixation of a Press-Fit Titanium Hip Joint Endoprosthesis, Clin. Orthop. 235: 195-206. However, it has been found that a substantial torque load results when the heel strikes the ground, when the patient climbs stairs, or, in general, when the hip joint is extended from the flexed position. If one studies the phylogenetic and ontogenetic development of the femural neck, it becomes clear that this xe2x80x9cheel-strike phasexe2x80x9d causes a large retrotorsional moment to be applied to the neck of the femur. The design of the prosthesis must take this torque into account.
Thus far, success in applying the force proximally to the metaphysis has only been achieved by using bone cement, as described in Draenert, K., and Draenert, Y., 1992, Forschung und Fortbildung in der Chirurgie des Bewegungsapparates 3: Die Adaption des Knochens an die Deformation durch Implantate [Research and Advances in Locomotor System Surgery 3: Using Implants to Adapt the Bone to Deformations], Munich: Art and Science. In 1986, M. A. R. Freeman asked why the neck of the femur should be resected (Freeman, M. A. R., 1986, Why Resect the Neck?, J. Bone Joint Surg., 68B: 346-349); increased rotational stability was discussed but not implemented in the design, since the importance of the anatomical structures had not yet been recognized. Thus, the resulting stem was straight and did not exhibit right-left symmetry.
However, it was found that it was precisely the anatomical structures of the proximal femur that were responsible for the bone""s rigidity, torsional strength, and ability to withstand high flexural loads. Thus, all of the anatomical structures, right down to the finest structural reinforcement provided by the spongiosa, need to be provided in the highly integrated overall design.
The present invention relates to a uniquely configured stem for a femoral insert of a hip prosthesis. The stem has a parabolic outer surface at its proximal end and a dorsal indentation that is straight along the length of the stem. The dorsal indentation changes in depth along the stem length with the deepest indentation at the proximal end. The configuration optimizes the transfer of force from the prosthesis to the bone.